Summary Viruses infect organisms in all kingdoms of life, posing a challenge to all hosts of recognizing the presence of a foreign invader. Little is known about how simple metazoans, such as the model organism C. elegans, recognize infection by viruses or how downstream signaling pathways or transcription factors are activated following virus infection. Studies in model organisms have given rise to many fundamental paradigms in biology. For example, seminal discoveries of caspase cell death pathways and RNA interference (RNAi) were first made in C. elegans. A new virus infection model in C. elegans was recently established that capitalizes on the discovery of Orsay virus (ORV), a positive strand RNA virus that is the first and to date only virus capable of naturally infecting C. elegans. With this system, it is now possible to exploit the robust genetic tractability of C. elegans to define the mechanisms by which C. elegans recognizes and signals the presence of RNA virus infection as well as the viral determinants that trigger such signaling. These studies will provide novel insights into the evolution of self versus nonself discrimination and could also lead to identification of novel antiviral pathways that are broadly conserved. Little is known mechanistically about viral sensing in C. elegans. Multiple members of the RNAi pathways have proven to be important in antiviral immunity including DRH-1, a dicer related helicase that shares some similarity to the antiviral sensor RIG-I. ORV infection also induces a specific transcriptional response in C. elegans, including a set of genes that is induced >100 fold specifically in the intestine, which is the primary site of ORV infection. Notably some of these genes are not transcriptionally activated following ORV infection in drh-1 mutants, demonstrating a role for drh-1 in both RNAi and transcriptional regulation. However, it is currently not known what additional C. elegans genes are important for inducing this transcriptional response. Neither are the specific ORV determinant(s) recognized by the host to trigger the response known. The goals of this proposal are to (1) Identify C. elegans genes that recognize or signal in response to ORV infection using an unbiased forward genetic screen and (2) Define the viral determinant(s) that are recognized by C. elegans to induce a transcriptional response to ORV infection.